Superlattice approach to doping infinite-layer nickelates

The recent observation of superconductivity in infinite-layer ${\mathrm{Nd}}_{1-x}{\mathrm{Sr}}_x{\mathrm{NiO}}_2$ thin films has attracted a lot of attention, since this compound is electronically and structurally analogous to the superconducting cuprates. Due to the challenges in the phase stabilization upon chemical doping with Sr, we synthesized artificial superlattices of ${\mathrm{LaNiO}}_3$ embedded in insulating ${\mathrm{LaGaO}}_3$, and we used layer-selective topotactic reactions to reduce the nickelate layers to ${\mathrm{LaNiO}}_2$. Hole doping is achieved via interfacial oxygen atoms and tuned via the layer thickness. We used electrical transport measurements, transmission electron microscopy, and x-ray spectroscopy together with ab initio calculations to track changes in the local nickel electronic configuration upon reduction, and we found that these changes are reversible. Our experimental and theoretical data indicate that the doped holes are trapped at the interfacial quadratic pyramidal Ni sites. Calculations for electron-doped cases predict a different behavior, with evenly distributed electrons among the layers, thus opening up interesting perspectives for interfacial doping of transition-metal oxides.

Figure 1

Left: Sketch of the pristine (reoxidized) ${\mathrm{LaNiO}}_3$ (blue)- ${\mathrm{LaGaO}}_3$ (gray) superlattice with $m=8$, $n=4$ stacking sequences grown on a (001) ${\mathrm{SrTiO}}_3$ substrate. Right: Structure stabilized by layer-selective reduction of the nickelate slab with ${\mathrm{CaH}}_2$. We label individual layers within the nickelate slab by $L1\text{−}L4$. The dashed horizontal line indicates the mirror plane.

Figure 2

(a) XRD patterns of a set of pristine, reduced, and reoxidized ${\mathrm{LaNiO}}_{3(2+\delta )}/{\mathrm{LaGaO}}_3$ superlattices including the (001) and (002) reflections of the ${\mathrm{SrTiO}}_3$ substrate. The asterisks mark peaks arising from the $K_{\beta 1}$ reflection of the main substrate peak. The data in panel (a) have been translated vertically for clarity. (b) Resistivity of the same set of samples.

Figure 3

quad simulations of the XRD data around the (002) ${\mathrm{SrTiO}}_3$ substrate reflections for different crystal structures of the (a) pristine ${\mathrm{LaNiO}}_3$ and (b) reduced ${\mathrm{LaNiO}}_{2+\delta }$ superlattices. In panel (b), the simulation of the proposed quadratic pyramidal (black box inset) interfacial nickel coordination yields a better agreement with the experimental results than the square planar (gray box inset).

Figure 4

Polarization-averaged XAS spectra [$I_{\mathrm{average}}=(2I_{E\perp c}+I_{E\parallel c})/3$; see Fig. 5] of the energy range covering (a) $\mathrm{La}{M}_{5,4}$ and $\mathrm{Ni}{L}_{3,2}$ edges, (b) of the $\mathrm{Ga}{L}_{3,2}$ edges (smoothed because of the lower flux at these high energies), and (c) around the $\mathrm{O}K$ edge. In each panel, data for pristine and reduced samples are offset for clarity. All spectra were measured in total-electron yield detection mode.

Figure 5

Polarization-dependent XAS, measured in total- fluorescence yield for pristine and reduced samples across the $\mathrm{Ni}{L}_{3,2}$ absorption edges, where the $\mathrm{La}{M}_4$ lines were previously subtracted. The top and middle panels show the spectra taken with x-ray polarization $E\perp c$ and $E\parallel c$, respectively. A sketch of the geometry is shown in the inset of the top panel. The bottom panel shows the linear dichroism defined by the difference of intensities $I_{E\perp c}-I_{E\parallel c}$.

Figure 6

(a) Partial density of states of the inequivalent ${\mathrm{LaNiO}}_{2+\delta }$ layers in the eight-layer stack with respect to electronic orbitals near the Fermi level (except for the $\mathrm{La}5d$ pDOS, which is averaged over all La sites) from DFT calculations. (b) Contour plot of Wannier functions with planar $x^2-y^2$ symmetry in the $ab$-plane (upper panels), and axial $3z^2-{\mathbf{r}}^2$ symmetry in the $ac$-plane (lower panels) for interface- ($L1$) and innermost layer ($L4$). Black (red) dots indicate the position of Ni (O) ions. The dashed line marks the contour value of 0.05.

Figure 7

Layer- ($L1\text{−}L4$) and orbitally resolved $\mathbf{k}$-integrated spectral function $\mathrm{A}\left(\omega \right)$ from DMFT for interaction values $U=8\mathrm{eV}$ and $J=0.8\mathrm{eV}$ at $T=1/50\left({\mathrm{eV}}^{-1}\right)\approx 232\mathrm{K}$. The columns correspond to different total electron fillings, where negative (positive) $\mathrm{\Delta }N$ corresponds to hole (electron) doping. The insets show the orbital-dependent real part of the DMFT self-energy ${\mathrm{\Sigma }}^{\prime }\left(i{\omega }_n\right)$ on the Matsubara frequency axis $[{\omega }_n=(2n+1)\pi k_{B}T,n\in \mathbb{Z}]$. The energy range and the color code for the plots are identical to the ones used in Fig. 6.

Figure 8

(a) Layer- and orbital-resolved distribution of the density $\mathrm{\Delta }n$ at the inequivalent Ni sites. (b) Layer-resolved spin density $\rho \left(S\right)$ as determined by projecting the many-particle density matrix into the basis of the $S^2$ operator.

Figure 9

Negative-${\mathrm{C}}_s$ HRTEM images of the $m=8,n=4$ (a) pristine and (b) reduced superlattices measured in ${[1-10]}_{\text{pc}}$ projection. The interfaces between ${\mathrm{SrTiO}}_3$ substrate (yellow) and ${\mathrm{LaNiO}}_{3(2+\delta )}$ (blue) -${\mathrm{LaGaO}}_3$ (gray) layer stacks are clearly shifted in the reduced sample due to the $d_{001}$ contraction upon oxygen removal. Schematic atomic model is overlaid on the HRTEM image in panel (a). (c) Values of the $d_{001}$-spacing in pristine and reduced superlattices measured from the HRTEM images (a,b), where each data point represents the average value of 15 unit cells along the ${\left[110\right]}_{\text{pc}}$ axis. For comparison we show the corresponding values from the DFT structure relaxation.

Figure 10

Layer- and orbital-resolved imaginary part of the self-energy. The lines are annotated with the corresponding quasiparticle renormalization factor where there is a quasiparticle peak present in Fig. 7.